1. Field of the Invention
The invention relates to a method for manufacturing a semiconductor device, more particularly to a method for manufacturing a semiconductor device using an epitaxy growth method, an epitaxial substrate for use in the method, and a semi-finished semiconductor device.
2. Description of the Related Art
Generally, in the fabrication of a semiconductor device using the epitaxy growth method, an epitaxial substrate is needed to permit growth of an epitaxial layer thereon, and the epitaxial substrate is selected to have a lattice constant substantially matching the lattice constant of the epitaxial layer. However, some of the substrate cannot offer good thermal conductivity. For example, in a III-V groups solar cell, epitaxial elements which are epitaxially grown on an epitaxial substrate are primary made from gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs), gallium indium phophide (GaInP), aluminum gallium indium phosphide (AlGaInP), and GaAs related materials, and thus a lattice matching material for the epitaxial substrate is preferably gallium arsenide (GaAs). However, since the GaAs substrate has a low thermal conductivity, the resulting solar cell is likely to suffer from high working temperature, low efficiency, and short service life due to heat generated during operation and sun light irradiation, etc. On the other hand, if a material with high thermal conductivity is selected, the same has lattice mismatching problem with the epitaxial material for the epitaxial elements, which leads to a large amount of defects in the epitaxial structure, thereby lowering the quality of the epitaxial structure.
To solve the aforementioned problems, it has been proposed to use a substrate made of a material having a lattice constant matching that of the epitaxial layer. The substrate is then removed after growth of an epitaxial structure, and the grown epitaxial structure is transferred to another substrate with a higher thermal conductivity. In this manner, the thermal conductivity of the resulting semiconductor device is raised and the removed substrate can be recycled after a proper cleaning process so as to reduce overall manufacturing costs.
Referring to FIGS. 1 and 2, there is shown a conventional method for making a semiconductor device, in which a substrate transferring step is involved. As shown, a buffer layer 12 is formed on a gallium arsenide substrate 11. Then, a sacrificial layer 13 having a lattice constant matching that of a semiconductor epitaxial material to be grown and a high etching selectivity is then formed on the buffer layer 12. Thereafter, an epitaxial structure 14 of the semiconductor epitaxial material is epitaxially grown on the sacrificial layer 13. Subsequently, a thermal conductive substrate 15 is attached to a surface of the epitaxial structure 14 remote from the gallium arsenide substrate 11. Finally, the sacrificial layer 13 is destroyed by wet etching to enable separation of the epitaxial structure 14 from the gallium arsenide substrate 11, thereby producing a semiconductor device with the thermal conductive substrate 15.
Because the sacrificial layer 13 is removed by wet etching using a chemical etchant, the overall thickness thereof is preferably not too thin. Otherwise, the etching rate would be relatively slow to undesirably prolong the time period of the manufacturing process. Moreover, the epitaxial structure 14 might be damaged after long-term immersion in the chemical etchant. On the other hand, if the thickness of the sacrificial layer 13 is increased in order to shorten the time period required for etching, stress between the sacrificial layer 13 and a surface of the epitaxial structure 14 would be increased, thereby resulting in more lattice defects in the epitaxial structure 14. Furthermore, the large thickness and stress may cause stress relaxation, which may lead to a great amount of dislocation defects in the epitaxial structure 14 and higher surface roughness, thereby reducing the efficiency of the semiconductor device.
In addition, the sacrificial layer 13 can only be etched from lateral sides using the chemical etchant. The etching rate is very low.
Referring to FIG. 3, in order to raise the rate of removal of the sacrificial layer 13, there is proposed another method for manufacturing a semiconductor device, wherein etching holes 16 which extend downwardly from the epitaxial structure 14 through the sacrificial layer 13 and the buffer layer 12 are initially formed to enable the chemical etchant to enter the etching holes 16 and reach the sacrificial layer 13 so that the contact surface between the chemical etchant and the sacrificial layer 13 is increased to raise the rate of removal of the sacrificial layer 13. This method, however, provides limited enhancement in the rate of removing the sacrificial layer 13.